for the taste of it
Clearly, yes.
Some people ingest semen during sexual intercourse.
Most Islamic scholars frown on oral sex as a general matter, so there is rather firm opposition to swallowing semen. Of course, what Muslims do in the privacy of their own homes is a different matter entirely.
The rifampin in the semen will not cause any problems, but their semen may not be safe to ingest. Rifampin is an antibiotic given for many reasons, including STD's. If the rifampin is being given for an STD then it would not be safe to ingest the semen, not because it has the rifampin in it, but because it may still have infectious particles in it.
Girls do not produce semen, but yes, if you are free of diseases, another girl can safely ingest your secretions.
Consuming your own semen will do you no harm.
Full of vitamins ingest away, assuming no std's aids etc.
Conserving semen has no benefits. It is a myth (wrong belief) that conserving semen is good.
Animal
Iv always been interested in asking males if they ingest their semen. All of them always say they do not. If you want to eat your semen, do it. It can not hurt you. Its comprised mostly of glucose, and half the chromosomes for your unborn children. It could possibly be looked at like cannibalism since is does contain human DNA in which case doing such would be frowned upon. As it stands now...in the words of Nike...JUST DO IT.
old blood in the semen, usually due to straining while having a bowel movement or injury to testcles. If this continues, see a doctor, but usually clears up on it's own.